The present invention relates to an expansion valve of a refrigerating cycle including a compressor with variable capacity.
As the compressor in a refrigerating cycle of an air-conditioning system of an automobile directly is driven by the engine, the speed of the compressor cannot be controlled individually. In order to obtain proper refrigerating ability, speed compressors with variable capacity are used (e.g., compressors of the inclined plate type, the rotary type or the scroll type). The capacity of the compressor is changed in correspondence to the pressure of the inhaled refrigerant or the inhalation pressure.
The expansion valve in a refrigerating cycle is controlled based on variation of the temperature and the pressure of low-pressure refrigerant supplied from the evaporator. Since the outlet of the evaporator directly communicates with the inlet of the compresor, the pressure of the refrigerant exiting the evaporator is equal to the inhalation pressure of the compressor. Any variations of the pressure at the outlet of the evaporator directly affect the inhalation pressure of the compressor. If the capacity of compressor is changed due to a variation of the inhalation pressure, and if then the expansion valve opens and closes suddenly, corresponding to the variation of the capacity of the compressor, the outlet pressure of the evaporator quickly varies. The degree of the variation of the capacity of the compressor is also amplified by variations of the inhalation pressure, since the compressor has to follow the inhalation pressure variations. This amplification leads to an undesirable hunting effect over a certain period of time, e.g. seconds up to several minutes in the flow of the refrigerant.